The enemy below

By Austin Hagan, Auburn University

Plant-parasitic nematodes, which are tiny, unsegmented roundworms, are widely recognized across the United States as damaging pests affecting cool- and warm-season turfgrasses. These free-living pests that feed in or along the surface of fibrous roots, are, however, not evenly distributed across the country. Because nematodes reproduce fastest in warm, moist soils, destructive outbreaks are most likely to occur in states with long growing seasons. Consequently, much of the Deep South — especially Florida — as well as southern California and Arizona are ground zero for damaging nematode outbreaks on recreational, commercial and residential turfs. When grown on lighter soils or greens with a high sand content, nearly all warm-season and some cool-season turfgrasses are inviting targets for plant-parasitic nematodes. Once established, you cannot eradicate nematodes, but you can minimize the damage they cause.

While it's not unusual to extract nematodes from soils across the northern half of the United States, plant-parasitic nematode numbers in these regions are rarely high enough to noticeably damage most lawns and sports turf. The occasional green or tee grown on a sand-based mixture is the most notable exception to this rule.

THE USUAL SUSPECTS

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Although 10 to 12 nematode genera are known to parasitize turfgrass root systems, only a few are common and destructive pests. Sting (Belonolaimus longicaudatus) nematode is the most feared of these pests on cool- and warm-season turfgrass. Fortunately for most turf managers and superintendents, this nematode is largely restricted to the coastal plain of the Deep South, Arizona and Southern California. Even relatively small populations are likely to damage a bermudagrass green, tee, lawn or sports turf. Other warm-season turfgrasses are also susceptible to attack by the sting nematode.

Other nematodes that often reach damaging population levels on turfgrasses include the lance (Hoplolaimus spp.) and ring (Criconenella spp.) nematodes. Like the sting nematode, you can find damaging populations of the lance nematode primarily on bermudagrass greens and tees, as well as other warm-season turfgrasses. While ring nematode has long been considered a significant threat to centipedegrass lawns (particularly in Florida), extremely high populations have also recently been tied to the decline and death of bentgrass greens on several Deep South golf courses. In addition, root-knot nematode (Meloidogyne spp.), which can attack a variety of turfgrasses, has been found at damaging levels on an increasing number of greens planted to dwarf and super-dwarf bermudagrass selections, as well as on St. Augustinegrass lawns. Again, the highest root-knot populations have been seen on sandy soils or high-sand-content greens. Occasional damage, particularly to greens and tees, has also been associated with high soil populations of the stubby-root (Paratrichodorus and Trichodorus spp.), sheath (Hemicycliophora spp.), lesion (Pratylenchus spp.) and stunt (Tylenchorynchus spp.) nematodes.

THE EVIDENCE

A thinning canopy, yellowing leaves and slow growth rate are among the most visible symptoms of nematode damage on turf. Since nematode populations across a green or lawn are never uniform, patches of nematode-damaged turf are randomly scattered and vary greatly in number, size and shape. On hot summer days, wilting of irregular patches may occur under little or no moisture stress, particularly on low-cut greens and tees. On ring nematode-damaged bentgrass, the patches of wilted turf, which may encompass much of a green, usually die within a few days. When cut for harvest, nematode-damaged sod simply falls apart and must be scrapped. Finally, nematode-damaged greens, tees and lawns do not respond well — if at all — to your applications of a fertilizer, water or pesticides.

Unfortunately, some or all of the above symptoms can be attributed to poor fertility, low soil pH, compaction, drought stress, root-feeding insect pests, several patch diseases and other sources of turf stress. Feeding activity of damaging nematode populations does significantly reduce the fibrous root mass. Damaged roots are often discolored, short and have few fibrous roots. Tiny galls or knots along with the females may appear on the roots of root knot-damaged bermudagrass or St. Augustinegrass. However, it is almost impossible for you to see this damage without the aid of a low-power microscope and specialized training.

DIAGNOSING TURF

An assay of soil collected from the damaged area(s) to determine the identity and numbers of plant-parasitic nematodes is the only accurate method of diagnosis. Diagnosis of nematode injury based solely on the previously-mentioned non-specific symptoms is next too impossible. An assay of soil collected from the damaged area(s) to determine the identity and numbers of plant-parasitic nematodes is the only accurate method of diagnosis. You may be able to eliminate other causes of turf decline by carefully examining the damaged turf as well as by submitting samples for a soil fertility assay and mineral analysis of the foliage. Ideally, you should submit several soil samples collected from both “bad” and “good” areas for both nematode and soil fertility assays, which will greatly speed up the diagnostic process.

For golf courses and other intensively managed lawns, a yearly sampling program is the best method for spotting plant-parasitic nematodes before appreciable damage occurs. Routine sampling is particularly important in those areas of the Deep South and Far West where the sting and other nematode pests are most common. Take 5 to 10 cores to a depth of 3 to 4 inches from the target green or lawn with a soil sampling tube in an X or W pattern. Because nematodes are rarely uniformly distributed, separately sample any areas of discolored, thinned or slow-growing turf and note their location on a map of the green or lawn. For best results, you should collect soil samples for nematode assay when nematode populations peak in mid-summer through mid-fall. Because populations of plant-parasitic nematodes usually collapse over the winter months, results of assays performed during this time period and the early spring are often inaccurate and misleading.

Most land-grant universities have a service laboratory that will process soil for a nematode assay for a small fee and will provide control recommendations based on the results of that assay. For further information, contact your local county extension agent or the turfgrass pathologist at your state's land-grant university. In addition, a number of private laboratories also provide nematode soil assays as a service to their customers.

COMING TO TERMS

For sports turfs, commercial and residential lawns, management practices that minimize or reverse the effects of stress offer the only real hope for maintaining the quality of a nematode-damaged turf. Recommended practices that promote vigorous root growth and enhance turf quality include deep, infrequent watering, a balanced fertilization program, correct mowing height and timely aerification. In order to help maintain good turf cover on nematode-damaged greens and tees during the hot summer months, you should raise mowing height in late May or early June. Also, you should implement an aggressive core-aerification program to offset the damage caused by nematodes on both bentgrass and bermudagrass greens and tees. Finally, syringing has proven quite helpful on really hot, sunny days to prevent the sudden wilting and death of ring nematode-damaged bentgrass greens.

Right now, there is no silver bullet for controlling nematodes on turf. Golf course superintendents and sod producers have the option of several costly but sometimes marginally effective nematicidal treatments. Nematicides for turf use fall into two broad catagories: pre-plant fumigant or post-plant contact nematicides.

You can use pre-plant fumigant nematicides such as methyl bromide, Vapam and Busan (metam-sodium), Telone II (1,3 dichloropropene) or Basamid (dazomet) to speed up the establishment of greens, tees and sod fields and temporarily suppress populations of damaging nematodes and noxious weeds. Methyl bromide, which has been tagged as a major cause of ozone depletion in the atmosphere, will be phased-out over the next few years. To stretch out supplies, chloropicrin (Terr-O-Gas 67) is being added to methyl bromide formulations. All soil fumigants are restricted-use pesticides and may only be purchased and applied by certified applicators. Methyl bromide is typically applied only by licensed custom applicators and is the most costly of all pre-plant fumigants.

Curfew, a formulation of 1,3 dichloropropene, is currently being evaluated under an experimental-use permit for the control of nematodes on established bermudagrass in Florida. Apparently, bermudagrass on greens, tees and fairways has good tolerance to Curfew, but researchers have not clearly established the sensitivity of other warm and cool-season turfgrass. You apply this product to bermudagrass with a series of coulters and shanks to a depth of 5 inches. The injection slits heal over in about 2 weeks and the turf fully recovers shortly thereafter. If Curfew passes muster with EPA, it may be on the market in the near future.

Within the past few years, the number of post-plant nematicides cleared for use on established turf has been whittled down to the emulsifible concentrate (3 EC) and granular (10G) formulations of Nemacur (phenamiphos). Use of both Nemacur formulations is restricted to golf courses by certified applicators. The granular formulation Nemacur 10G is the most widely used of the two. When applied in April or May and again in July at the rate of 2.3 pounds of product per 1,000 square feet of treated area, Nemacur 10G gives good control of most nematode pests, including the sting and lance nematode. To speed the movement of Nemacur through the thatch and soil profile, core-aerify, spike or Hydro-Ject the turf prior to treatment.

Like many organophosphate pesticides, Nemacur is highly toxic to fish and waterfowl. To prevent runoff into nearby streams or ponds, you must irrigate Nemacur-treated turf immediately with ½ inch of water. Also, keep waterfowl off of treated turf until after the foliage dries.

Although the biological control of nematodes has not been extensively studied, products such as Clandosan, Neo-trol and DiTera are cleared for use on turf. Clandosan, which is a combination of soybean meal, urea, and processed shrimp or crab shells, is designed to increase the activity of bacteria and fungi that breakdown chitin. Because chitin is the main component in the “skin” of a nematode, the enzymes produced by these microbes should kill many of the nematodes in the root zone. You can broadcast Clandosan, which is a granular product, at rates of 45 to 140 pounds of product per 1,000 square feet. Neo-trol [Nematrol], which is composed of ground sesame, contains several water-soluble chemicals that are toxic to nematodes. You can apply Neo-trol as a broadcast spray over residential, commercial and recreational turfs at rates of 230 pounds of product per acre. Di-Tera, which is the dried fermented and solutes of a strain of the fungus Myrothecium verrucaria, is cleared for use as a foliar spray at rates of 13 to 100 pounds of product per acre.

THE VERDICT

The results of the limited numbers of university studies assessing these and other biological products for nematode control on warm- and cool-season grasses have been very mixed. Dramatic reductions in the numbers of plant-parasitic nematodes with biologicals are few. Also, little improvement in turf quality or sward density has also been obtained with biological products. Basically, the jury is still out. Clearly, additional field trials need to be conducted to assess their efficacy against common nematode pests and their overall impact on turf quality. However, you can use biological nematicides in all turf settings, especially residential and sports turfs, where Nemacur 10G and Nemacur 3 applications are not allowed. So in situations where there are no other treatment options, using a biological may be the best way to go. Golf course superintendents on an organic fertilizer program may also want to evaluate a product such as DiTera or Neo-trol on a green or two, as well as monitor nematode populations and turf quality in treated and untreated plots. Also, with EPA's continued scrutiny of organophosphate products, we need to seriously consider alternatives.

Nematodes are tough pests to deal with on established residential, recreational and commercial turfs. Timely sampling is the best method for finding and then tracking damaging nematode populations. Regardless of the situation, good management practices will minimize the damage caused by nematodes. You should consider Nematicide treatments only when you have found damaging populations and good management procedures have failed to suppress damage.

Austin Hagan is an Extension Specialist and professor at Auburn University (Auburn, Ala.)